This invention relates to a process for starting and for subsequently laying a submergible elongate structure upon the bed of a body of water. More particularly the invention relates to a process for starting and for subsequently laying a pipeline upon a water bed.
During underwater pipelaying operations, in which pipe is laid from a floating marine vessel onto the bed of a body of water, it is imperative that the elastic bending limit of the pipeline should not be unduly exceeded. In this connection if the bending radius of the pipeline becomes too small, as is possible in the case of a pipeline which is permitted to sink from a barge under its own weight without restraint, the pipeline may become permanently deformed, cracked, or otherwise rendered unusable.
Prior art attempts to provide support for a pipeline, as it is being lowered, include the use of buoyant devices which are positioned within the water in a manner to support and guide the pipeline along a proper bending curvature. These buoyant devices have been used in combination with ramps or stingers which project form the vessel and underlie the pipeline, during at least part of its submerged descent. Examples of previously known stinger or ramp devices are disclosed in U.S. Pats. Nos. 2,910,835 to Timothy, 3,280,571 to Hauber et al., 3,390,532 to Lawrence, 3,487,648 to Lawrence, 3,507,126 to Rochelle et al., 3,566,609 to Smith and 3,606,759 to Lochridge.
While such devices have proved to be highly satisfactory the initial capital cost may in some instances be prohibitive. Moreover, large ramps may be difficult to manipulate during some laying operations, such as in deep water.
Recently, dynamic tensioning units have been developed which apply, from the marine vessel, a longitudinal restraining force directly upon the pipeline as it is payed out to replace or supplement the function of the previously known stinger devices. For a full understanding of dynamic tension pipelying methods and apparatus reference may be had to U.S. Pats. Nos. 3,390,532 to Lawrence and 3,487,648 to Lawrence.
Notwithstanding the high degree of success achieved by dynamic tension pipelaying operations, care must be taken when using these units to prevent damage to the pipeline coatings particularly where it is necessary to apply high tension forces. Moreover, when applying tension to a pipeline, it is necessary that there be a reaction force within the pipeline. Hence, unless the pipeline is anchored, tensioning units are ineffective during an initial laying operation. Moreover, as a pipeline is being initially laid, it is often desirable to connect the free end thereof to a hook-up facility of some type, such as a submerged pipe installation or a riser. Once the coupling is made, it is desirable that initial tension on the pipeline be kept at a minimum to prevent undue stress from being applied to the coupling joint.